Oligonucleotides are useful in therapeutic, diagnostic, research and nanomaterials applications. The use of naturally occurring nucleic acids (e.g., unmodified DNA or RNA) for therapeutics can be limited, for example, because of their instability against extra- and intracellular nucleases and/or their poor cell penetration and distribution. Additionally, in vitro studies have shown that properties of antisense oligonucleotides such as binding affinity, sequence specific binding to the complementary RNA (Cosstick and Eckstein, 1985; LaPlanche et al., 1986; Latimer et al., 1989; Hacia et al., 1994; Mesmaeker et al., 1995), and stability to nucleases can be affected by the absolute stereochemical configurations of the phosphorus atoms (Cook, et al. US005599797A). Therefore, there is a need for new and improved oligonucleotides and oligonucleotide compositions, such as, e.g., new antisense and siRNA oligonucleotides and oligonucleotide compositions.